ENVIRONMENTAL ENGINEERING
May 22-23, 2008 The 7th International Conference
Faculty of Environmental Engineering Vilnius Gediminas Technical University Saulėtekio ave 11, LT-10223 Vilnius, Lithuania Phone: +370 5 2745090; Fax.: +370 5 2744731; e-mail:
[email protected]
PRELIMINARY SHORT-TERM TEST OF METHANE OXIDATION CAPACITY IN POROUS MATERIALS – EVALUATION OF RELIABILITY Witold Stepniewski and Malgorzata Pawlowska Lublin University of Technology, Faculty of Environmental Engineering, Nadbystrzycka 40B Str., 20-618 Lublin, Poland E-mail
[email protected] [email protected]
Abstract. Landfill gas (LFG) poses the treat to the environment. The most profitable way for LFG utilization is using it as a energy source. It is impossible when the CH4 concentration in biogas is below 30%. However, under these conditions microbial oxidation of CH4 in porous media could be a solution. A choice of filling of the biofilter for LFG purification requires the preliminary estimation of CH4 oxidation capacity. The test should be simple and quick, but it should also give reliable results. The results of short-term tests of the methanotrophic activity of two types of organic materials (without pre-incubation in over-atmospheric level of CH4) were compared with the values of the activity measured after the long-term incubation of these materials at high methane concentrations. The aim of the examination was the evaluation of reliability of a short-term test and finding a correlation between the results of short and long-term studies. Keywords: methane oxidation, landfill gas, methods, biofiltration.
1.
range of 30-50 m3h-1 exclude the economically grounded use of the biogas as a energy source. Microbial oxidation of this gas leads mainly to the production of simple inorganic compounds, i.e. CO2 and H2O, according to the reaction:
Introduction
One of the ways to the minimization of the landfill influence on the atmosphere is biofiltration of the landfill biogas through porous materials, such as soils, composts, peats, wood chips, and mineral substrates. Some of the biogas components passing through the filling bed are utilized by specific microorganisms, which are able to transform them into the chemicals more safe for the environment. Biofiltration is useful for the removal of many organic compounds, especially malodors, some aliphatic and aromatic hydrocarbons, methane and inorganic compounds, i.e. NH3, H2S. It is a simple and cheep method of gas purification, which is especially effective for the removal of the gases of low concentrations and assures the complete elimination of the pollutants. When the concentration of the gas and the rate of the biogas flow through the biofilter are high the efficiency of its removal is not sufficient to eliminate its emission. In the case of methane the biofiltration is recommended when the concentration of this gas in the biogas is below 15% v/v [21]. According to [9] the concentration of CH4 lower than 35-40% and total gas production value below the
CH4 + 2O2 → CO2 + 2H2O Part of the carbon can be built into the methanotrophic bacteria cells, then the amount of produced CO2 is lower than stoichiometric ratio. This type of CH4 oxidation is called “assimilative pathway of oxidation”. The ratio of CO2 produced to CH4 oxidized in soils taken from vicinity of natural gas leakages varied in a wide range from 0.2 to 0.9 [10]. According to the above reaction it should be equal 1. The value of this ratio is an important indicator of the phase of microbial population growth. The lower is the ratio the more intensive is the growth of the methanotrophs, and vice versa. The ratio closer to 1 the more stabile growth of their population. However, this indicator can be taken into consideration only in pure cultures or in mineral materials. The high content of organic matter in the tested samples leads to CO2 production by decomposition of the organic compounds. Then, the amount of produced CO2 may
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values from the one unit to another is feasible but it gives only approximate values due to geometry effect and inhomogeneity of the bed. There is no standard method of methanotrophic capacity evaluation, both under static and dynamic conditions. Therefore, the comparison between the values found in the literature meets serious difficulties and it should be done cautiously. In every case the detail information about the method of the examination, soil sample mass and properties, preparation way, incubation conditions, etc. should be given. The aim of the study is the evaluation of the
be even several times higher than the amount of oxidized CH4. In the sand material with average organic matter content of about 0.6% w/w the ratio of CO2 production to the CH4 uptake ranged from 1.2 to 4.0 [17]. The evaluation of the capacity of soil materials to CH4 oxidation under landfill conditions has been the subject of detailed examinations since the 90’s of the XX century [3, 24, 11, 12, 2] although, there were papers concerning this problem in the 80’s [14, 15]. The aim of the examinations was to determine the actual activity of the samples taken from landfill cover or to evaluate the usefulness of various kinds of the material to methane biofiltration under landfill conditions. An assessment of methane oxidation capacity of a porous material may be performed in few ways. The methods can be classified (see Fig.1) according to the type of the gas supplying system (permanent flow dynamic method, temporary gas injections - static method) or to the incubation time (long and short time).
reliability of the short-term test and looking for the relationship between the results of short and long-term approach.
2.
Materials and methods
The experiments were carried out using two kinds of organic materials: a mature municipal solid waste (MSW) compost and commercially available artificial organic substrate – POKON used for seedlings growing. The compost was taken from ZUSOK (Communal Solid Waste Treatment Plant) in Warsaw. It was sieved through 4mm sieve before using in the experiments. The organic substrate was taken to the experiment without special preparation. The properties of the examined materials are presented in Table 1. Table 1. Properties of a municipal waste compost and an organic substrate POKON [13, 26] Property Actual water content (% of dry weight) Water holding capacity (% of dry weight) Bulk density (kg dm-3) Total porosity (%) Total organic carbon (TOC) (% of dry weight) TKN (% of dry weight) pH
Fig 1. Classification of the methods of methane-oxidizing capacity assessment
The most popular method of methane oxidation rate measurement is the long term, dynamic study carried out in column microcosms [12, 22, 7, 16, 17, 18, 20]. This type of the studies allows to monitor the changes of methanotrophic capacity with time, maintaining the landfill-like conditions. It makes possible to examine the samples from different depths of the layer and determine kinetic parameters of stabilized bed in static tests. But, the studies have to last least 1 month in order to establish steady-state conditions. The microbial processes are very sensitive to many factors. The parameters of soil porous material, concentration of substrates and their availability to microorganisms, temperature of sample incubation, etc. effect the results of the experiments. Thus, the choice of the method is very important and should take into account the intended aim of the examinations. The expected results should comprise the actual state of the system or its methanotrophic potential. Evaluation of the capacity could be performed with respect to the unit of mass or to the volume of the material, as well as to the unit of the surface area of the material bed. Conversion of the
MSW compost 25
POKON 187
80
345
0.62 76.4 10.7
0.43 88.0 40.7
0.85 7.6
0.94 5.5
The experiment was carried out in two stages. - First stage – start of the dynamic experiment in PVC columns filled with MSW compost and POKON substrate (each material in three repetitions) and performance of the static tests on the raw materials. - Second stage – static tests conducted on the materials taken from the columns (from the depth of 80 cm where the concentration of CH4 during whole was ca. 8%) after 6 months of dynamic experiment duration. The dynamic experiment was conducted in 6 PCV columns (see Fig 2.). Each column was equipped with valves, which made it possible to flush the mixture of methane, carbon dioxide and oxygen
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from the bottom. The profiles of gases distribution were analysed during the entire experiment. Gas samples and soil samples (when the steady state condition were established) were taken by sampling ports distributed at 10 cm intervals along the columns. Each port was plugged with a septum in order to take the samples of the gas from particular depths. The bottom of the columns was shaped in the form of a funnel, which allowed free outflow of an excess of water. The lower part of the column were filled with gravel to assure uniform access of methane to the filling material. A perforated plate was put on the gravel and 80 cm of the examined material was placed on it. The scheme of a single column is presented in Fig. 3. The experiments were carried out under laboratory conditions, at a temperature of 22±2 oC. Each material was examined in three repetitions. Fig 3. Scheme of single column used to assessment of CH4 oxidation capacity in the dynamic system
Scheme of the set up is presented in Fig. 4. The temperature of incubation was 22±2. The 150 µl headspace gas samples were taken from vials by gas tight syringe through the rubber plugs and analyzed by Shimadzu 14B Gas Chromatograph. A glass column with Porapak Q was used for determination of CH4 and CO2 concentrations. The Chroma X 2007 version 0.1b program was used for data processing. The experiment lasted about 24 hours. The “zero” samples were taken within first 10 minutes after vials plugging.
Fig 4. Scheme of the static test
Fig 2. Experimental set up for methanotrophic activity determination by dynamic method
Changes in CH4 and CO2 concentrations in the headspaces, dependent on time, were taken to the calculations of methanotrophic activity, defined as the amount of methane oxidized per unit of mass of material within unit of time.
The gas mixture (7.5% of CH4 + 7.5% of CO2 +17% of O2 and 68% of N2) was applied to the base of each column at a rate of 20 cm3min-1 for six months. The static tests were carried out on 0.5 g samples of materials (each material in three repetitions). The samples were placed in glass vials of 25.5 cm3 volume and tightly closed by rubber plugs. Then, the methane was injected into the vials by using a syringe to the initial concentration ca. 8%. During the gas injection the second needle was driven into the plugs in order to equilibrate the pressure inside the vials.
3.
Results and discussion
Decrease of methane concentration with time was observed in all vials. But the rate of the methane uptake varied significantly among particular samples. The comparison of methane concentration changes in both the raw materials and in the samples of the materials taken after 6 months of incubation under high CH4 concentration are presented in Fig. 5
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9 8
C H4 c once ntra tion [% v/v ]
could be expected higher values of methane oxidation activity (VCH4) were observed in the materials tested after incubation. The activity measured in incubated compost was equal to 168 cm3 kg-1h-1 and it was about 1.4 times higher than the activity of incubated organic substrate. Raw material were characterized by lower values of methanotrophic activity, equal to 52 and 86 cm3kg-1h-1 in organic substrate and compost, respectively. The values of methane oxidation activity found in the experiment were very high in comparison to the results obtained by other authors under laboratory conditions. The potential methanotrophic activities (Vmax - calculated from Michaelis-Menten equation) are presented in Table 3. The highest Vmax, equal to 304 cm3 kg-1h-1 were found by Wilshusen et al. [25] in compost prepared from municipal green waste mixed with manure examined in a laboratory study. It was about 1.8 times higher than the maximum values found in our experiment in incubated compost, and about 3.5 times higher than the value measured in organic substrate after incubation. In the comparison to other data given in Table 3 the values found in our experiment, especially in incubated materials were high. It was probably caused by non limited oxygen accessibility during the incubation phase. It should be noticed that the results given in Table 3 are not potential methanotrophic activity values, but just mean actual activities. The increase of methane oxidation activity during incubation measured in compost was about 2 – fold while the increase of this value in organic substrate was slightly higher and oscillated around 2.4 times.
raw compost incubated compost raw organic substrate incubated organic substrate Liniow y (incubated organic substrate ) Liniow y (raw compost ) Liniow y (raw organic substrate ) Liniow y (incubated compost)
10
7 6
R2 = 0,8277 5 4
R2 = 0,8139
3 2 1
R2 = 0,976
R2 = 0,9542
0 0
500
1000
1500
2000
Time [min]
. Fig 5. Changes of CH4 concentrations in headspaces observed in a raw (before incubation) and incubated (exposed on high CH4 concentration for 6 months) materials raw compost incubated compost raw organic substrate incubated organic substrate Liniow y (raw compost ) Liniow y (incubated organic substrate ) Liniow y (incubated compost) Liniow y (raw organic substrate )
7
6
CO2 concentration [% v/v]
5
R2 = 0,787
Table 2 Methanotrophic activity, CO2 production rate and the relationship between these two values calculated for each examined material
4
Material 3
R2 = 0,795 2
Raw compost Incubated compost Raw organic substrate Incubated organic substrate
R2 = 0,5271 1
R2 = 0,4946 0 0
500
1000
1500
2000
Time [min]
Fig 6. Changes of CO2 concentrations in headspaces observed in a raw (before incubation) and incubated (exposed on high CH4 concentration for 6 months) materials
CH4 oxidation activity (VCH4) cm3g-1 h-1 86
CO2 production rate (VCO2) cm3g-1 h-1
VCO2/V
22
0.26
169
46
0.27
52
26
0.5
122
125
1.03
CH4
ratio
Increase of CO2 concentration within time was also of concern (see Fig. 6). The highest slop of the trend line was observed in the case of incubated organic substrate what probably results from production of CO2 caused by the mineralization of
In all vials CH4 concentrations decreased linearly. The values calculated from the slops of the lines presented on Fig. 4 are gathered in Table 2. As it
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organic matter during long time of substrate aeration in dynamic experiment. Only in this material the rate of methane uptake (VCH4) was lower than the rate of CO2 production (VCO2) which was about 125 cm3 kg1 -1 h (see Table 1). The CO2 production rate in other examined materials was few time lower and ranged from 22 cm3 kg-1h-1 in raw compost, through 26 cm3 kg-1h-1 in raw organic substrate to 46 cm3 kg-1h-1 in incubated compost.
other processes related with decomposition of organic matter. The TOC content of this matter was about 4 times higher in organic substrate than in the compost. 4. Conclusions The results of examinations carried out on two kinds of materials (MSW compost and organic substrate) suggest that the short-term test could be used for preliminary assessment of methanotrophic activity in porous materials. Obviously, it does not give the real value of activity observed in steady state process which is reached after long time of incubation, but it could be helpful in selection of more suitable material. We observed, that the increase of methane oxidation activity during incubation measured in the compost was about 2 –fold and the increase of this value in organic substrate was about 2.4 –fold. The preliminary test confirmed the higher activity of methane oxidation in raw compost. It was about 1.6 time higher in this material than in raw organic substrate. The ratio of the methanotrophic activities measured after incubation in the compost to that of the organic substrate was about 1.4. The problem of “stability” of the ratio of methane oxidation activities obtained in short-term test and long-term study requires further investigations. It should be confirmed on many different materials.
Table 3 The potential methanotrophic activity (Vmax) of methane oxidation in landfill cover soils or in model studies, according to different authors Reference
Examined material
CH4 concentration [% v/v]
[cm3kg -1 -1 d.w. h ]
Soils of agricultural origin Sandy loam from landfill cover Loam from landfill cover Crushed expanded clay in biofilter, Sand and gravel materials, different depths of column Coarse sand soil from landfill cover Compost preapared from MSW and manure
0.005 - 3.0
4 – 36
[5]
